Philip Rivers last season became the 32nd quarterback in NFL history to post a passer rating of over 100 for a season.

But Rivers doesn’t quite know how he got to his rating of 105.5 – the highest ever posted by a Charger and the 13th-highest in NFL history.

“I know some of the things in the ratings,” Rivers said recently of the weighted mathematical formula the NFL and the Elias Sports Bureau uses to rank passers.

“But I do not know exactly how they are computed. I know yards per attempt is in there and you pay a penalty for interceptions. But the formula, I don’t know it.”

Few do, although the NFL has used its current formula – the ninth used to rate quarterbacks since the league started keeping official statistics 77 seasons ago — since 1973.

“It’s not perfect, but in general very good,” Steve Hirdt of Elias said of the NFL passer ratings. “The ratings are an easy subject for people to make fun of.

“But the system has been around for 36 seasons. And when you look at them, there is a correlation to the ratings and a team’s success.”

This season for example, the NFL’s five highest-rated passers – Drew Brees, Brett Favre, Aaron Rodgers, Peyton Manning and Rivers – play for teams with a combined record of 47-8.

And four of the 10 quarterbacks with the highest single-season passer ratings in NFL history led their teams to NFL championships.

But the NFL is quick to point out that the ratings are “passer” and not “quarterback” ratings.

Nowhere in the formula are considerations for won-loss record, red-zone success and fourth-quarter drives. Even statistics such as sacks, fumbles and rushing touchdowns are absent.

Basically, there are just four components of the NFL passer ratings – completion percentage, average yards gained per attempt, percentage of touchdowns per attempt and percentage of interceptions per attempt.

Passer ratings in the NFL have been based on those four categories since 1962. But it was in 1973 that the NFL, using the scoring of the Olympic decathlon as a rough guideline, decided to develop a mathematical formula to rank the passers.

“Between 1962 and 1973, passer ratings were based on how quarterbacks ranked in the four categories,” said Hirdt. “If you led the league in completion percentage you got a point. If you were 25th among starting quarterbacks in average gain per attempt, you got 25 points.

“The lowest score adding up the rankings in the four categories won the passing championship. The flaw in those rankings is that they were based on what everyone else was doing that particular season.

“The NFL wanted a system that would grade passers strictly on their performance.”

And a system that could compare quarterbacks from one season to the next, and one era to another.

The crux of the current system is found inside each of the four main categories.

The maximum rating possible is 158.3. But that is not a perfect score, since the system eliminates extremes at the top and bottom of performance.

For example, the maximum score in completion percentage is achieved with a percentage of 77.5 percent. One hundred percent is no better than 77.5 percent. At the other extreme, any completion percentage below 30 percent is worth zero points.

For equation purposes, the maximum points in each of the four categories is 2.375. To reach the rating, add the scores from the four categories. Divide that total by six. And multiply that by 100 to get the rating.

While Drew Brees reached the maximum score of 158.3 with his performance Monday night against the Patriots, the passer rating is designed to grade performance over a season.

“The system is designed to measure over a season or a career rather than a game,” said Hirdt, who called a score of 80 to 85 average, 90 to 100 good and anything above 100 excellent.

If there is an anomaly in the system it’s that it tends to favor quarterbacks in high-percentage, low-risk offenses that control the ball with their passing games. Passers in the 1970s and 1980s who regularly threw deeper down field – such as Dan Fouts – scored lower in the current system because their interception rates were higher and completion percentage lower.

The passer rating numbers have climbed over the past two decades with the evolution and derivatives of “West Coast” offenses.

“The big difference is the emphasis against turnovers,” said Hirdt, who authored baseball’s save rule. “As with a lot of statistics, when you cross generations the results can get a little weird.

“But there are statistics that drive strategy.”

Chargers head coach Norv Turner, who also admitted he didn’t know how the passer ratings are computed, disagrees that passer ratings might influence strategy.

“If a quarterback is worried about his rating, he’s worrying about the wrong thing,” said Turner, who also noted the passer rating is void of a way to judge “degree of difficulty.”

“In many areas, the passer ratings are a decent reflection and in some areas they are not very relevant,” said Rivers.

“I don’t put a lot of stock in them. Shouldn’t there be a calculation for wins and losses? It’s somewhat valid, but they don’t tell the whole story. Turning the ball over is big. The end all is wins and losses. I’d trade an 88 for winning a game.”

“I see them, but I don’t put a lot of value in them,” said Volek. “I think they probably establish some value to a quarterback’s performance over a season, but not in an individual game.

“I know if you throw a lot of touchdown passes, have a high completion percentage and gain a lot of yards per attempt, you are going to have a high rating. But throwing interceptions really hurts you.”

Charlie Whitehurst, the Chargers’ third quarterback, believes individual football statistics are far less valid than individual baseball statistics.

“In baseball, a lot of statistics are batter vs. pitcher, individual battles inside a team game. But in football, when you drop back to pass, 11 guys are involved. Actually, 22 guys are involved.

“The ratings don’t factor in blitzes, broken routes, dropped balls, fumbles, a lot of things. I guess there is some validity because a winning team’s quarterback usually has a good rating.

“But it’s not at all a concrete grading system. I don’t think you can have one.”